This invention relates to FM broadcasting systems and, more particularly, to an improved receiver for an FM stereophonic broadcasting system which utilizes companding of the stereo difference signal.
U.S. Pat. No. 4,485,483, the disclosure of which is hereby incorporated herein by reference, describes a stereophonic broadcasting system which utilizes companding of the difference signal and is compatible with existing FM receivers. As shown in FIG. 4 of the patent, stereophonically related audio frequency source signals L and R are matrixed to obtain stereophonic sum and difference signals M and S, respectively. At the transmitter, the difference signal is used to amplitude-modulate a first sub-carrier signal and at the same time is compressed and used to amplitude-modulate a second sub-carrier of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, double-sideband modulation is employed, with the frequency of the sub-carrier signal being sufficiently high to assure a frequency gap between the lower sidebands of the modulated sub-carrier signals and the sum signal. A conventional low-level phase reference pilot signal, lying within the aforementioned gap, is provided for detection purposes at the receiver. The composite baseband signal comprising the sum signal, the two modulated sub-carrier signals, and the pilot signal and which will sometimes be referred to herein as the "FMX" signal is frequency modulated onto a high frequency carrier for broadcast. The receiver includes a demodulator for deriving the sum signal, the usual difference signal S and the compressed difference signal S', and an expander for expanding the derived compressed difference signal. The expander may be of the type described in commonly assigned U.S. Pat. No. 4,602,381, the disclosure of which is also hereby incorporated herein by reference, which uses the difference signal S as a reference signal for controlling the expansion of the received compressed difference signal S' so as to cause the amplitude of the expanded difference signal to equal the level of the uncompressed difference signal. The expanded noise-reduced version of the difference signal is combined with the received sum signal to obtain the original audio frequency source signals L and R. In addition to improving the quality of the received signal, the system increases the broadcast coverage area over that of current biphonic service.
Although the "FMX" signal transmitted by the broadcasting system described in the aforementioned patents can be compatibly received with properly aligned conventional receivers, as is pointed out in commonly assigned patent application Ser. No. 030,223 filed Mar. 25, 1987 by Emil L. Torick, the the disclosure of which is hereby incorporated herein by reference, should the receiver fail to maintain the proper phase relationship between the pilot signal and the sub-carrier, there may be crosstalk of the compressed signal into the uncompressed difference signal and, depending on the direction of phase misalignment, may add to or subtract from the normal difference signal. If the phase error is negative, the crosstalk will be out-of-phase and may cause a perceived narrowing of the stereo stage width, and if the phase error is positive there may be an apparent widening of the stereo image.
The problem is exacerbated when the receiver is moving because of multipath disturbances caused by reception of directly transmitted signals as well as delayed reflections of the transmitted signal from man-made structure of the surrounding terrain. Depending on the delay intervals and the number of delayed reflections received, multipath can decrease the level of the received RF signal and cause noisy reception or, in some cases, complete signal dropouts. In addition to RF signal fading, the summation of the multipath signals at the receiver may distort the phase relationship between the pilot signal and the stereo difference signal, and with the inclusion of the compressed difference signal S' in the transmitted signal, momentary phase errors can result in bursts of crosstalk, as well as noise. Since the level of the compressed difference signal is generally higher than that of the uncompressed difference signal, if the phase error is such as to cause crosstalk summation of the two stereo difference signals loud bursts of sound may be heard.
According to the teaching of the aforementioned co-pending Torick application Ser. No. 030,223, the composite "FMX" signal can be made more compatible with conventional FM stereo receivers, particularly in the presence of multipath propagation, by adjusting the relative amplitudes of different portions of the audio frequency spectrum of the compressed difference S' in approximate correspondence with the response of the human hearing mechanism to sound loudness level. The significance of this modification of the compressed difference signal will be seen from FIG. 1 which illustrates the amplitude versus frequency spectrum of the uncompressed and compressed difference channels of the "FMX" signal described earlier. The compressed signal, illustrated in the lower curve, has the same upper and lower frequency limits as the uncompressed stereo difference channel, has an amplitude of 20 dB over the range from about 1,000 Hz to about 15,000 Hz, rolls off from about 1,000 Hz to about 100 Hz at a rate of about 6 dB per octave to a level of 0 dB, that is, to the level of the uncompressed difference signal, and remains at that level until the lower limit of the passband is reached. Stated another way, the compressed difference signal used to amplitudemodulate the quadrature carrier at the transmitter has the same amplitude as the uncompressed difference signal from the lower end of the illustrated passband up to a frequency of about 100 Hz, then increases to a level of about 20 dB at a frequency of about 1,000 Hz at which it is maintained for the balance of the spectrum. Alternatively, instead of the compression characteristic having a zero dB "shelf" over the frequency range between about 20 Hz and 100 Hz, it may continue to roll off at the 6 dB per octave rate, as indicated by the dashed line 8.
While the described modification of the ampitude/frequency spectrum of the compressed stereo difference signal makes the composite "FMX" signal more compatible with conventional FM stereo receivers, unless precautions are taken in designing the receiver for the composite "FMX" signal, under certain conditions the noise at the output of the receiver could be worse than at the output of a conventional receiver. That is to say, if the filter of the receiver's expander has a characteristic complementary to the encoding filter characteristic, the expander filter would boost all of the low frequencies and under certain conditions could boost them to a level at which they would be in excess of what they would be in a normal FM stereo receiver. Moreover, boosting of all low frequency signals could affect the location of the knee of the expander characteristic in such a way as to cause the expander to mistrack under certain disturbance conditions.
A primary object of the present invention is to provide a receiver for use in an "FMX" broadcasting system wherein the characteristic of the transmitted compressed difference signal corresponds approximately to the loudness response of the human ear.